The invention is based on a priority application EP03292257.7 which is hereby incorporated by reference.
The invention relates to a wavelength converter for binary optical signals.
Such wavelength converters are for example known from the U.S. Pat. No. 5,978,129. The wavelength converter disclosed therein comprises an interferometer structure, in particular a Mach-Zehnder interferometer. Such an interferometer substantially comprises a parallel connection of a first and a second semiconductor optical amplifier SOA between its input for a local signal and its output. The interferometer serves for generating the output signal by modulating the received local signal according to the modulation of a further received first input signal. This is achieved by providing said first input signal to said first or said second SOA. Explained in more detail, the modulation of the first input signal represents a modulation of power. When such a modulation of power is provided to one of said SOAs the gain thereof is modulated in response to the modulation of the power of the input signal. Because there is a linear relationship between the gain and the phase of such a SOA, the phase of said SOA also changes in response to the modulation of the input power. Simultaneously with the change of the phase of one SOA also the relationship of the phases between the two SOAs changes in such a way, that the phase difference between said two SOAs is about 180°.
In that way an output signal of said interferometer structure is generated by modulating the received local signal according to the modulation of the further received first input signal. The known wavelength converter further comprises a first pre-amplifier means for amplifying said first input signal before it is input to said interferometer structure. Finally, the wavelength converter comprises a control means for controlling the gain of said pre-amplifier means in response to the output signal such that the extinction ratio of the output signal is kept maximal.
When the wavelength converter is embodied as described above and in particular in the way that the interferometer structure only receives one input signal besides the local signal the interferometer structure is operated in a standard mode.
However, the interferometer structure may also be operated in a differential mode at higher data rates than in the standard mode. The operation at higher data rates is achieved in the differential mode by providing a second input signal to the second SOA of the interferometer and by modulating the modulation of the second input signal onto the local signal, too.
As known from the U.S. Pat. No. 5,978,129 the control means of the wavelength converter serves for keeping the extinction ratio of the output signal maximal. Now, one could think of providing a copy of said known control means for additionally controlling said second signal in a same manner in order to keep the extinction ratio of the output signal of the wavelength converter maximal in the differential mode. However, that does not work because the sensitivity of the first and the second SOA to polarization are different and leads to an unsatisfactory operation of both SOAs and consequently to an undesired output signal having not a maximal extinction ratio.
Starting from that prior art it is the object of the invention to improve the controlling of the interferometer structure of a known wavelength converter such that a satisfactory operation of the SOAs of the interferometer is achieved, even if the interferometer structure is operated in a differential mode.